passivering

Passivering, also known as passivation, is the formation of a protective, adherent oxide film on a metal surface that reduces its reactivity to the surrounding environment and slows corrosion. The process is driven by the thermodynamics of oxide formation; the film is typically a few nanometers to tens of nanometers thick and is self-healing: if damaged, the underlying metal can re-oxidize to reform the film.

Commonly studied in stainless steel, aluminum, titanium, and nickel-based alloys. In stainless steel, chromium-rich oxide (Cr2O3) forms a passive layer that protects against corrosion in many environments. In aluminum, aluminum oxide (Al2O3) provides protection; titanium forms TiO2. The protective film is usually electrically insulating, greatly reducing ionic transport, which slows anodic dissolution.

Passivation can be induced or enhanced by chemical or electrochemical treatments. Chemical passivation uses oxidizing acids (e.g., nitric acid) or organic acids to remove surface contaminants and promote oxide formation. Electrochemical passivation involves applying a potential to promote oxide growth.

Environmental factors influence passivation. Proper pH, low aggressive anions (like chlorides) and moderate temperatures favor stable films. Highly aggressive environments, high chloride concentration, or mechanical damage can lead to film breakdown and localized corrosion (pitting) if the passive film is compromised.

Limitations and considerations: the passive film is not permanent; it can degrade under wear, abrasion, or certain chemical exposures. Maintenance and compatible alloy selection are important for long-term protection.

See also: corrosion, oxide layer, surface treatment.